The tread of a tire is the portion of the circumferential surface of the tire that contacts the road surface. Traditionally, automobile, motorcycle, truck, bicycle, and airplane tires have a series of grooves that intersect the tire tread dividing it into several parts. On most race cars, “slick” tires are used. These tires have a single tread that extends from one sidewall to the other and has no grooves. Standard tires (i.e. tires with grooves) have a strong grip of the road surface so long as (i) the tire is not over-worn and (ii) the weather is dry and warm. In wet weather conditions they tend to aquaplane or hydroplane because water on the surface of the road interferes with the tread contacting the road surface. In Formula 1 racing, special grooved tires are fitted on the race cars during wet conditions. The pattern of grooves serves to disperse water and somewhat avoid aquaplaning and hydroplaning. There are two types of Formula 1 wet weather tires: “wet” and “extreme wet”. The former are used when it is raining and the latter when there are puddles of standing water on the track.
Other tires (e.g., those used on passenger vehicles and on commercial trucks, airplanes, etc.) have multiple treads that are separated by complex grooves. The grooves do not improve traction under dry conditions, but they do serve to avoid hydroplaning and aquaplaning in wet weather. The grooves also provide an easy means for measuring the age of the tire and whether the tire is safe to drive on. As tire treads wear, the depth of the grooves is shortened thereby diminishing the ability of the grooves to effectively disperse water.
Most drivers examine their vehicle's tires just by looking at the treads. However, there are more accurate ways to measure the depth of the grooves, the age of the tire, and whether the tire is safe. A new tire typically has 10/32″ groove depth. A groove depth of 8/32″ is still fine to drive. A depth of 6/32″ is an ample depth, but it may be difficult to stop in the snow. With a depth of 4/32″, it may be difficult to stop in the rain. A tire is considered “bald” when the groove depth is 2/32″. This tire is not safe to use even in dry conditions.
One way to measure the groove depth is to use a coin. Place a penny into several tread grooves at least 15″ apart circumferentially on the tire. When part of Lincoln's head is covered, it means the tire has more than 2/32″ of tread thickness. If the top of his memorial is always covered, then the tire has more than 6/32″ of tread thickness. However, it is difficult for most people to remember these rules.
There are plenty of tire tread gauges available, but they are relatively large and difficult to use.